1. Field of the Invention
This invention relates to a digital voice transmission system and more particularly to a digital voice transmission system wherein voice data to be transmitted are selected from requested voice data and the digital voice data are compressed and transmitted whereas the compressed data are decompressed and the voice is reproduced by a digital voice reproduction radio reception apparatus with a display unit.
2. Description of the Related Art
In a conventional digital voice transmission system, a voice message is digitized into and transmitted as binary data between the transmission side and the reception side. In a paging protocol such as the FLEX, since it has a binary data transfer function, a voice message is transmitted as binary data.
As shown in FIG. 1, data to be transmitted include an ID code for identification of a digital voice reproduction radio reception apparatus on the reception side, a voice compression data identification code for identification that the data to be transmitted are voice compression data, and digital voice compression data.
When the size of the voice compression data exceeds the amount which can be transmitted at one time, the voice compression data are divided and transmitted divisionally.
As shown in FIG. 2, data to be transmitted include an ID code for identification of a digital voice reproduction radio reception apparatus on the reception side, a message ID for identification of a message, a division number indicative of a number in order of a division, a voice compression data identification code for identification that the data to be transmitted are voice compression data, and digital voice compression data.
In the following, division of voice compression data by the digital voice transmission system described above is described.
Division of voice compression data is performed by compressing voice data of L bytes obtained by digital sampling and making an object of transmission and dividing data of K bytes obtained by the compression in units of an amount (M bytes here) which can be transmitted at one time as seen in FIG. 3. Then, the voice compression data obtained by the division are transmitted in the data format shown in FIG. 2 to a digital voice reproduction radio reception apparatus.
In the following, a digital voice reproduction radio reception apparatus employed in the digital voice transmission system described above is described.
The digital voice reproduction radio reproduction apparatus in the present conventional example includes, as shown in FIG. 4, reception antenna 120, reception circuit 121 for demodulating a signal received by reception antenna 120, decoder 122 for decoding the received data demodulated by reception circuit 121 and discriminating whether or not the decoded data are a signal destined for the radio apparatus itself, ID memory 123 in which an identification number of the radio apparatus is stored, RAM 124 for storing the received data, CPU 128, CPU program ROM 125 in which an operation program for CPU 128 is stored, DSP 129 for voice decompressing the voice compressed data, DSP program ROM 130 in which a program for operation of DSP 129 is stored, digital to analog converter (DAC) 131 for converting the digital voice signal into an analog voice signal, amplifier (AMP) 132 for amplifying the voice signal, band-pass filter (BFP) 133, loudspeaker (SPK) 134, and operation switch (SW) 135.
In the following, operation of constructing voice compression data when reception of divisional transmission data of the digital voice reproduction radio reception apparatus having the construction described above is missing is described.
The operation of constructing voice compression data when reception of divisional transmission data of the conventional digital voice reproduction radio reception apparatus is missing, will be explained below with reference to FIG. 5.
First, it is discriminated whether or not voice data received are a message destined for the apparatus itself (step S111).
If it is discriminated in step S111 that the received voice data are a message destined for the self apparatus, the number of the division received is placed into N (step S112).
Then, it is discriminated in step S112 whether or not the value of N into which the number of the division has been placed is 1 (step S113).
If it is discriminated in step S113 that N is 1, then since this signifies that the data received are new voice compression data (step S114), 1 is placed into M (step S115), and the received, divided voice compression data PartData are placed into VoiceData (step S116).
On the other hand, if it is discriminated in step S113 that N is not 1, then it is discriminated whether or not the current transmission is transmission of a division next to the last division number M received in the past (step S117), and if (N=M+1) stands, then it is discriminated that no division is missing (step S118) and the PartData are added to VoiceData (step S119). On the other hand, if (N=M+1) does not stand in step S117, then an error in reception is determined and the receiving operation is ended (step S120).
However, the operation of the conventional apparatus described above has such disadvantages as described below.
(1) Since an entire voice message inputted on the transmitter side of the voice message is voice compressed after digital sampling and then transmitted, the amount of information increases in proportion to the length of the voice message.
Here, a radio communication system having a low transmission rate such as a system for a paging service does not have a capacity sufficient to accept the capacity of digital voice data. For example, in a FLEX system or an ERMES system which is one of paging protocols, the transmission capacity is approximately 6,000 bps.
Therefore, it is considered that transmission of a long voice message or an increase of the number of times of origination to the receiver side increases the capacity of transmission data until it exceeds the transmission capacity of the system, and reduction in number of transmitters or a delay in transmission is invited inevitably.
(2) When transmitting digital voice data by a radio channel, an error or drop of data may possibly occur depending upon the state of the radio channel. In this instance, since the voice compressed data are handled as binary data, either it is impossible to restore data following the portion at which the drop of data has occurred into voice or the voice message is rendered discontinuous.
As a result, when the voice message is reproduced on the receiver side, the voice is produced but discontinuously and cannot be heard readily. Further, if reception of a voice data start code representative of the start of transmission of voice compression data misses, then reception of the voice compression data becomes impossible.